1. Field
The present invention relates to the field of automated pipetting apparatuses and more particularly to automated pipetting apparatuses which employ an electrically conductive reference body.
2. Description
International Patent Application publication number WO 91/16675 describes an automatic pipetting device of the type indicated above. For the purpose of determining reference values of the coordinates in each of the movement directions (X, Y, Z directions) of the pipetting needle, a surface of an electrically conductive reference body is scanned by an electrically conductive pipetting needle. Electrical contact occurring when the surface of the reference body is touched by the pipetting needle is detected by an electrical circuit.
The disadvantage of this known device is that repeated contact between the surfaces of the reference body and the sometimes dirty outer wall of the pipetting needle, promotes deposit formation on the scanned portions of the reference body's surfaces. There deposits may be electrically conductive or non-conductive, depending on the nature of the pipetted liquids.
If the deposits are electrically conductive, they result in inaccurate detection of the position of the scanned surfaces. As a result, the determination of the reference coordinate values necessary for accurate control of the pipetting needle transport is adversely affected.
If the deposits are electrically non-conductive, they prevent the intended electrical contact between the pipetting needle and the reference body. In such a case because contact between the pipetting needle and the reference body is not detected, the pipetting needle reaches the scanned surface and it continues to be pressed against the reference body by the transport device. The corresponding mechanical load may cause the pipetting needle to bend, making it unsuitable for further use in the analyzer. It is therefore an object of the invention to provide an analyzer which does not demonstrate the above disadvantages.
The Invention also relates to a method for achieving the above objective in the automatic transport of the pipetting needle of an automatic pipetting device.
According to the invention, the first part of the above stated problem is solved by an analyzer, which is characterized in that it comprises the following means:
a circuit for measuring the electrical capacitance between the pipetting needle and the reference body, and PA1 means for determining the coordinates of a reference position of the pipetting; needle in at least one of the horizontal transport directions (X and Y directions), said means being adapted so to move the transport device of the pipetting needle that the pipetting needle is moved to a position near each of the two outer surfaces of the reference body which are disposed perpendicularly to the transport direction, the pipetting needle being moved in each case towards a surface of the reference body until the electrical capacitance between the pipetting needle and the reference body reaches a predetermined value corresponding to a specific distance between the pipetting needle and the surface of the reference body. PA1 the electrical capacitance between the pipetting needle and the reference body is measured, and PA1 to determine the coordinates of a reference position of the pipetting needle in at least one of the horizontal transport directions (X and Y directions), the pipetting needle is moved by the transport device in opposite directions in each case to a position near each of the two outer surfaces of the reference body which are disposed perpendicularly to the transport direction, the pipetting needle in each case being moved towards a surface of the reference body until the measured value of the electrical capacitance between the pipetting needle and the reference body reaches a predetermined value corresponding to a specific distance between the pipetting needle and the surface of the reference body.
According to the invention, the second part of the above stated problem is solved by a method characterized in that:
A major advantage of the inventive analyzer and method is that the scanned surfaces of the reference body need not be contacted by the pipetting needle. Specifically, surface scanning is contact-free. In accordance with the present invention, no deposits can form on the scanned surfaces of the reference body. Thus, unwanted deposits cannot impair the accuracy and reliability of the inventive method, or lead to deformation of the pipetting needle.
Another advantage of the inventive analyzer and method is that correct and reliable operation of the device is guaranteed even if the pipetting system is filled with an electrically conductive liquid.
In a preferred embodiment of the inventive analyzer, the electrical resistance of the liquid contained in the tubing connecting the pipetting needle to the metering system is greater than a predetermined limit. This is achieved by suitably choosing the dimensions (diameter, length) of the tubing connecting the pipetting needle to the metering system. The advantage of this embodiment is that possible disturbances due to the electrical conductivity of the liquid in the pipetting system in combination with fluctuations in the electrical environment of the pipetting needle specific to the apparatus, cannot affect the function of the analyzer.